1. Field of the Invention
The field of the invention is that of the back-lighting of passive viewing screens also called LCDs for “Liquid Crystal Displays”. These screens are light modulators and require an external lighting source in order to operate.
2. Description of the Prior Art
In a certain number of applications, in particular in the aeronautical field, these screens are used by day and at night. Consequently, the lighting source must possess a high luminance range so as to ensure both correct daytime contrast under strong sunshine and a faintly luminous night-time image so as not to hinder the pilot's nocturnal vision. Thus, luminance ranges of the order of 1000 to 10 000 may be specified.
Technically, to achieve these high ranges, use is made of control signals modulated in terms of duty ratio, also called “PWM” for “Pulse Width Modulation”. These periodic signals comprise, during each period, a variable activation time. However, the specified luminance range may be greater than the range of the PWM control signal provided. For example, the range of the PWM signal may be limited to 100 whereas the required range is of the order of 1000.
For certain lighting sources, control by duty ratio turns out to be sufficient. Mention will be made notably of fluorescent lamps of “HCFL” (“High Cathode Fluorescent Lamp”) or “CCFL” (“Cold Cathode Fluorescent Lamp”) type. Indeed, when the activation time is very small, having regard to the technical nature of these sources, the light emitted is not proportional to the activation time but is much smaller than the latter whereas, when the activation time is greater, the light emitted becomes proportional to the activation time. For example, for an activation time corresponding to 1% of the period of the PWM, the quantity of light emitted will be 0.1% of the possible maximum, whereas, for an activation time corresponding to 50% of the period of the PWM, the quantity of light emitted will be close to 50% of the possible maximum. Thus, naturally, the sought-after increased luminance range is obtained.
However, certain lighting sources like light-emitting diodes or LEDs have very low response times. Having regard to their performance in respect of dimensions, luminous efficiency and lifetime, LEDs are increasingly used to achieve lighting sources for display screens. In this case, the previous effect is no longer present. If the light-emitting diodes are solely controlled by the PWM signal, the luminance emitted is directly proportional to the activation time of the PWM, no longer making it possible to obtain the sought-after effect, that is to say a high brightness range.
To alleviate this drawback, the modulation of the luminance of the LEDs is achieved either by modulating the amplitude of the current which passes through them, or by modulating the activation time over a given period by a PWM signal, or by combining the two modulations to obtain a very high depth of modulation. Technically, to carry out this modulation of the amplitude/modulation of the activation time distribution, use is made of an arithmetical and logical calculation function which works on digital signals. FIG. 1 represents a digital control device using this principle. This device 1 comprises a digital controller 2 which receives a luminance setting CL. This controller 2 generates two digital signals. The first signal is a temporal signal SPWM modulated in terms of duty ratio having a determined activation time, dependent on the luminance setting. The second signal SA-N is a control signal for the current passing through the array of light-emitting diodes. It is transformed into an analog signal SA-A by means of a digital-analog converter 3 or “DAC” and then applied to the electronic control circuits 4 for the LED array 5. The device can optionally be supplemented with a slaving device making it possible to finely adjust the luminance emitted by the diodes. It is represented by a dotted arrow in FIG. 1.
However, this technical solution may exhibit certain drawbacks. In the aeronautical context, even if these calculation resources are justified by other needs, the PWM/amplitude distribution calculation function is subject to the most constraining procedures of development and certification of RTCA/DO-254 type, entitled “Design Assurance Guidance For Airborne Electronic Hardware” or RTCA/DO-178 type, entitled “Software Considerations in Airborne Systems and Equipment Certification”.
Moreover, on account of problems of obsolescence related to the gradual disappearance of fluorescent lamps, equipment manufacturers are tending to replace back-lighting based on fluorescent lamps with lighting units based on LEDs. Now, as has been seen, fluorescent lamps are controlled by a simple PWM signal. In these cases, the equipment manufacturer or the aircraft manufacturer does not want to introduce modifications of the existing numerical calculation functions so as to avoid any re-certification of the viewing device or to add any, necessarily complex, digital circuit carrying out the PWM/amplitude distribution calculation.